Method for vapor growing ternary compounds

ABSTRACT

Ternary epitaxial films are grown from a gaseous mixture consisting of mercury, cadmium, and tellurium mixed with an inert or unreactive gas such as hydrogen. The mixture is heated to a temperature to prevent binary combinations and then rapidly cooled to the point of supersaturation by flowing the mixture through a thermal gradient having equithermal lines substantially parallel to the growth surface of a substrate.

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METHOD FOR VAPOR GROWING TERNARY COMPOUNDS BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates tosemiconductor devices in the form of ternary compounds and particularlyto a method and apparatus for vapor growing epitaxial films usingelements and compounds from the II-VI valence groups.

2. Description of the Prior Art In the prior art of vapor growingepitaxial films, it was considered essential to use disproportionationor other chemical reactants such as the halogens and their compounds.These were' considered to act as transport agents to the elementalreactants from which the semiconductor films are made. Unfortunately,for many applications, the resultant films contained a proportion of thehalogen element and thereby disturbed their molecular composition.

In a copending application of Carpenter, McDermott, Manley, Riley, Ser.No. 763,307, filed on Sept. 27, 1968, and assigned to the assignee ofthe present invention, it was shown that epitaxial films can be grownfrom the vapor phase without the presence of a disproportionationreactant, and that stoichiometric films can be achieved by condensationof the film constituents directly from the vapor phase to the solidstate. In that application, the system used one of the constituentreactants (e.g., Hg) as the transport agent and the apparatus wasdesigned as a closed system. Such a system, while useful for producinghigh grade ternary epitaxial films would have some limitation for largerscale production requirements. Using the mercury reactant in a largescale system would require excessively elaborate apparatus to maintainthe essential temperature and pressure conditions necessary to avoidside reactions detrimental to the growth process. Overpressure controlof mercury to effect the ternary combination in the closed system, andfurthermore, uniform crystal film composition over a large depositionarea, were difficult to achieve.

SUMMARY OF THE INVENTION The broad object of this invention is toprovide a method and apparatus capable of vapor growing ternaryepitaxial films at larger production rates.

It is a specific object of this invention to provide a method andapparatus for vapor growing ternary epitaxial films consisting of Il-Vlelements having uniform composition over a larger deposition area toproduce larger epitaxial film crystals.

It is a further specific object of this invention to provide a methodand apparatus for vapor growing epitaxial films of mercury cadmium, andtellurium which are useful as infrared detector devices and particularlyinfrared mosaic detectors.

It is also an object of this invention to provide a method and apparatusfor vapor growing epitaxial films of mercury, cadmium, and tellurium atrelatively low pressures and temperatures without usingdisproportionation reactant agents for growth and carrier control.

The above, as well as other objects, are attained in accordance with thepractice of this invention by forming a gaseous mixture consisting ofgrowth reactant gases from elements of the Il-VI valence groups and aninert transport gas such as hydrogen, argon, and mixtures thereof. Inthe preferred form, the constituent gases are added separately toindividual streams of the carrier gas, after which the individualstreams are combined and mixed to form a uniform distribution of theternary elements and the carrier gas. During the combining step, heat isapplied to set a temperature level high enough to prevent binaryreactions to occur. The mixture is then rapidly cooled to simultaneouslysupersaturate each of the growth constituents forcing them to condensesimultaneously on the growth surface of a seed crystal substrate. It isa feature of this invention that the rapid cooling is effected bypassing the ternary gas and carrier mixture through a thermal gradienthaving equithermal lines substantially parallel with the growth surfaceof the substrate.

The apparatus for practicing the present invention is of the open tubetype, that is, the maximum pressure within the apparatus does not exceeda pressure much above atmospheric. In the preferred embodiment, theapparatus comprises plural source furnaces connected in parallel to acommon carrier gas source. Source materials volatilized to theappropriate temperature and pressures are added to the gas streams ofthe carrier gas in the source furnaces. A mixing furnace having a commoninput from the source furnaces provides a uniform distribution of thereactant gases and the carrier gas for feeding to a reaction furnace.The mixing furnace comprises heating means operable simultaneously withthe mixing action to maintain the temperature of the mixture at a levelwhich inhibits binary reactions within the mixing chamber.

A growth substrate is mounted on a pedestal adjustably movable withinthe reaction chamber. A cooling means supplies air coolant to thesubstrate. Heating coils, one of which is movable, surrounds thereaction chamber to produce high temperatures necessary to maintain theternary mixture in its vapor phase until it is very close to thesubstrate growth surface. A flat equithermal profile is achieved byseparating the coils to provide a heating gap along the reaction chamberin the region of the deposition substrate. Unreacted gases pass from thereaction chamber, are condensed and the carrier gas passed into theatmosphere. With such an apparatus, continuous high rate film growingmay be practiced, pressures within the apparatus are low therebyassuring greater safety, and relatively large epitaxial films withuniform composition are obtained. An additional advantage of this systemresides in the fact that the source materials may be elemental in natureand when processed through the system are essentially being distilled.This insures freedom from contaminants such as copper which may bepresent in source materials even of the highest purity.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional schematicof a vapor growing apparatus for practicing the present invention;

FIG. 2 is a sectional view of a mixing chamber used in the apparatus ofFIG. 1;

FIG. 3 is a sectional detail of the reaction chamber of the apparatus ofFIG. 1 illustrating a thermal map for the reaction chamber for aparticular setting of heating coils for the reaction chamber; and

FIG. 4 is a graph of a thermal profile of the thermal gradient of FIG. 3taken along a center line 44 shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. I, theimproved vapor growing apparatus comprises source furnaces 10, I1, and12 connected in parallel between a source 13 of an inert carrier gas anda mixing furnace 14, which in turn is connected to a reaction furnace 15connected to apparatus 16 for venting the inert gas to the atmosphere.Source furnace 10 comprises a quartz chamber 17 wound with a heatingcoil 18. A supply of elemental cadmium 19, preferably in the form ofpellets in a quartz boat, as shown in the above-mentioned copendingapplication, is located within the heating zone established by coil 18.A current supply and regulating means of suitable type (not shown) whichis independently operable, is connected to heating coil 18 to maintaintemperature levels to effect volatilizing of the elemental cadmium intothe hydrogen gas stream as it flows through chamber 17. The channel forsupplying hydrogen gas to chamber 17 comprises tube 20 connected tochamber 17 via airtight seal 21, and through flow meter 22 and flowvalve 23 to a common flow line 24.

Source furnace 11 comprises a quartz chamber 25 wound with heating coil26 electrically connected and controlled in essentially the same manneras coil 18 of furnace 10. A supply 27 of elemental tellurium, preferablyin pellet form in a quartz boat. or the like, is located within theheating zone of coil 26 to be volatilized and added to a hydrogen gasstream flowing through chamber 25. The carrier gas channel to furnace 11comprises a tube 28 connected by an airtight seal 29 to chamber andthrough flow meter 30 and flow valve 31 to supply line 24. Sourcefurnace 12 comprises a T-shaped quartz tube 32 having one branchconnected by an airtight seal 33 to tube 34 and through flow meter 35and valve 36 to supply line 24. The second branch of chamber 32 isconnected to a mercury supply well 37. Liquid mercury 38 is fed bygravity from an external reservoir 39 through connecting tube 40 to well37. An electrical coil 41 which is wound entirely around the well 37 aswell as the entire junction area of tube 32 is electrically connected tocurrent source and regulating means of a suitable type for vaporizingthe mercury at predetermined temperature and pressure levels foraddition to a hydrogen gas stream flowing through tube 32. Theconnection of tube 32 to well 37 is preferably made long and the windingof coil 41 is such as to allow a measure of preheating of the mercuryvapors prior to their addition to the hydrogen gas stream in tube 32.The flow of hydrogen gas from source 13 is suitably measured forregulation means such as bubble column 42, or the like, connected tosupply line 24.

In its preferred form, the mixing furnace 14 comprises a cylindricalquartz chamber 43 entirely wound with a heating coil 44 which iselectrically connected to suitable current source and regulating means(not shown) which maybe independently operable to maintain temperatureof the mixing chamber 43 at levels to assure proper constituent control.As best seen in FIG. 2, the mixing chamber 41 has s single tube input 45connected in common with the outputs of chamber 17, 25 and tube 32 ofsource furnaces 10, 11, and 12, respectively. To effect thorough mixingand uniform distribution of the constituent elements in the hydrogencarrier gas stream, mixing chamber 43 is provided with a series ofbaffles 46-49 arranged to produce eddying within the hydrogen stream asit leaves tube 45. While various types of baffling can be provided andwhile the number of baffle elements may vary, the baffles 46-49, as seenin FIG. 2, are four in number and are preferably integral protrusionsformed from the walls of chamber 43. Such baffles are formed by heatingto soften the quartz walls. Then, using wedge shaped tools, havingstraight leading edges, the walls are impressed inwardly with thestraight and sloping surfaces as shown, with the innermost edgespenetrating the chamber to a point in line with the center line ofinputtube 45. As seen in FIG. 2, the baffles 46 and 47 are opposed at spacedlongitudinal positions. Baffles 48 and 49 are similarly constructed andshaped except that they are oriented 90 from the positions of baffles 46and 47. The longitudinal spacing of the baffles is a function of rate ofgas flow, but is in the preferred case set to produce eddying in 3dimensions thereby assuring complete and thorough mixing to effectuniform distribution of the constituent gases throughout the carrier gasstream. When forming the baffles 46-49 in the manner described, caremust be exercised to assure that the upper edge of the baffles aresubstantially diametrical, including the parts adjacent the walls ofchamber 43. Otherwise, some flow will occur around the baffles unimpededand produce inadequate mixing.

The reaction furnace 15 comprises a cylindrical quartz reaction chamber50, a pair of coaxial heating coils 51 and 52 wound thereon, and a meansfor supporting a growth substrate 55 at a selectable growth siteposition within the chamber relative to the heating coils. The reactionchamber 50 is preferably designed with a removable cylindrical section53 which is provided with a central opening and joins with the rest ofthe reaction chamber at airtight seal 54. The substrate supportcomprises a cylindrical pedestal tube 56 which is inserted through thecentral opening of bottom section 53. Seal ing means, such as O-rings57, are provided between pedestal tube 56 and chamber section 53. Agrowth substrate 58 is attached by suitable mans such as spring clip 59.Cooling means comprises a silver heat sink cylinder 60 inserted withinpedestal tube 56, and tube 6], connected through flow meter 62 to an aircoolant source. Both the heat sink 60 and spring clip 59 structures maybe other than the type shown in the above-mentioned copendingapplication. A viewing port 63 is provided in reaction chamber 50 in thegeneral area of the desired growth site. Venting of the carrier gas fromthe system is provided by tube 64 connected through valve 65 to a coldtrap 66. if the carrier gas is to be burned when vented, as in the casewhere hydrogen gas is used, an ignition device, such as coil 67, may beused. The system is also connected to a vacuum pump from tube 64 throughtube 68 and valve 69.

In the preferred embodiment of this invention, the heating coils 51 and52 are connected to separate current source and regulator means. Thispermits flexibility in controlling the heating effects necessary tocontrol the thermal gradients essential for vapor growing ternarycompounds. In addition, heating coil 52 is wound in such a manner thatit is movable longitudinally along reaction chamber 50. By the movementof coil 52, a separation 70 is provided which produces a thermal gap inthe heating field of the coils 51 and 52. By varying this gap, thethermal gradient profile, and thus the composition control of film grownon substrate 58 is obtained.

In order to operate the apparatus of FIG. I to vapor grow epitaxialfilms, certain preliminary procedures are necessary. Substrate 58 isselected to have the desired crystal structure then polished andcleaned. A suitable crystal would be a monocrystal of Cd Te of a nominalthickness of 10 mils cut from an ingot along the crystallographic plane.Cleaning and polishing may be performed as described in theabovementioned copending application. The selected crystal 58 is thenattached to the end of pedestal 56 by spring clip 59 and inserted withinreaction chamber 50 through the opening in bottom section 53 (which hasbeen assembled and sealed at 54) using O-rings 57 to effect reactionchamber sealing. Similarly, source materials 19 and 27 of cadmium andtellurium, respectively, are placed in heating zone position withinsource furnaces l0 and 11 and liquid mercury 38 adjusted to the desiredlevel within furnace 12 after which the reservoir 39 is blocked to closethe mercury feed system.

Following these procedures, and with all external joints firmly sealed,the system is evacuated to an initial pumpdown level through valve 69 toeliminate oxidizing gases and impurities. Hydrogen gas which flows fromsource 13 is then turned on and vented through the apparatus toatmosphere where it is ignited by coil 67. At the same time, the vacuumpump is stopped and valve 69 closed. A back-etch operation of substrate58 may then be performed according to well-known techniques to furtherpolish the growth surface of substrate 58. This may be done in theapparatus of FIG. I by turning on the reaction furnace 15, mixingfurnace 14, and the cadmium and mercury source furnaces l0 and 12.Source furnace ll is not turned on. For the back-etch operation, coil 52is moved virtually adjacent the coil 51, thereby eliminating gap 70, andboth are energized to a common temperature level. For example, with asubstrate ofCd Te, furnace 12 has a temperature of 300 C. and mixingfurnace l4, and source furnaces l0 and 12 have temperature settings of850 C., 370 C., and 315 C., respectively. Heating takes place for aperiod of 15-20 minutes, or until the substrate surface 58 is observedthrough aperture 63 to take on a glossy appearance.

Following the back-etch operation, coil 52 of reaction furnace 15 ismoved along chamber 50 to the desired separation of gap 70 and coils 51and 52 energized to the desired operating temperature levels for vaporgrowing. At this point in the operation streams of hydrogen gas enrichedwith cadmium gas and mercury vapor, flow through the system, are mixedin furnaces l4 and passed through to the reaction chamber 50. Since agrowth reaction does not yet take place, mercury and cadmium willcondense in the lower portion of chamber 50 and collect in bottomsection 53. Lastly, the source furnace ll is now turned on to volatilizetellurium from source 27 into the hydrogen gas stream flowing in chamber25. At the same time, cooling air is supplied to heat sink 60 to dropthe temperature of substrate 58 to the desired film growing level. Withall sections of the system thus operating, a crystaline ternaryepitaxial film becomes deposited on the upper surface of the substrate58. After a predetermined operating time to obtain the desired filmgrowth, the source furnaces and 11, and the mixing and reaction furnacesl4 and are turned off. The source furnace 12, however, is allowed toremain on after all the other parts of the system are turned off toallow the mercury vapor pressure to remain within prescribed levels inreaction chamber 50 to prevent mercury from being volatilized from thegrown film after air to heat sink 60 is cutoff. Furnace 12 continues tooperate until the reaction chamber 50 reaches a temperature ofapproximately 100 C. for Hg Cd Te films, then it is shut off and thesystem opened to the atmosphere.

Using the above operating procedures, epitaxial films of Hg Cd Te weregrown using hydrogen as the carrier gas with a flow rate of 60cc./minute, reaction chamber coil gap setting of 1 cm. and the followingtemperature settings:

Source Furnace l0 (cadmium) 305 C. Source Furnace ll (tellurium) 430 C.Source Furnace l2 (mercury) 3l0 C. Mixing Furnace l4 850 C. ReactionFurnace l5 (coil 5]) 800 C. Reaction Furnace l5 (coil S2) 530 C,Substrate 480 C.

lt should be noted that a minimum substrate temperature must bemaintained to promote epitaxy or single crystal growth and preventdendrite growth which can be a problem at substrate temperatures muchbelow 450 C. and the source temperatures mentioned above. ln thepreferred form of practicing this invention, stoichiometry is controlledin the growing layer by fixing the cadmium source temperature and, thus,its overpressure and then varying the amount of tellurium in the gasstream. When the amount of tellurium is the same as cadmium due to therelatively low substrate temperature, just Cd Te would be formed andthere is very little Te available for reaction with Hg. By adding Te inexcess of the cadmium present, some free Te is available to react andincorporate Hg in the growing crystal lattice. With the substratetemperature sufficiently low, the supersaturation of Te and Hg in thegas phase is sufficient to cause a complete reaction on the substrate.Under the conditions described, a rise of substrate temperature of 50C., while not appreciably affecting the Te-Cd reaction, is capable ofaffecting the quantity of Hg chemically reacting in the growing crystal.Thus, chemical composition is controlled by adjusting the ratio ofconstituents in the gas phase and by controlling the substratetemperature.

In accordance with this invention, the growth of ternary Hg Cd Teepitaxial films requires cooling the substrate 58 at the minimumtemperature and yet locating it as close to the mixing temperature (800C.) to thereby produce a very sharptemperature gradient as close to thesubstrate as possible. This prevents depletion of Cd and Te byspontaneous nucleation prior to arrival at the substrate. An example ofa thermal profile for a gap setting of 1 cm. with temperatures of coils51 and 52 set at 800 C. and 530 C. respectively, and a substratetemperature of 480 C. and a flow rate of cc./min. is shown in FIGS. 3and 4. Curve 71 illustrates the thermal profile taken along the centerline through reaction chamber 50 through heating zone of coil 51 to thegrowth surface of substrate 58. As there illustrated, the temperature ofthe gas mixture changes from over 600 C. to below 500 C. within a spaceof approximately 1 cm. to a point of about 0.5 cm. from the substratesurface. In passing through this sharp a gradient, vapors of cadmium,tellurium, and mercury become supersaturated and, since they are veryproximate the growth surface of substrate 58, condensation occursthereon.

As further illustrated in FIG. 3, the equithermal lines above the growthsurface of substrate 58 are substantially parallel with the growthsurface which is preferably flat and the temperature across the growthsurface is uniform over substantially the entire surface. Since filmgrowth is a function of substrate temperature, uniformity of substratetemperature assures uniformity of composition over the growth surface.This type of thermal profile is achievable by adjusting the separation70 between coils 51 and 52 to effectively produce a gap in the thermalfield in the region of the substrate 58. By contrast, with a single coilwound on the reaction chamber, the thermal profile would haveequithermal lines of curvilinear shape of substantially larger radiusextending from end to end of the substrate surface. Thus, a temperaturegradient exists across the growth surface. Thus, composition would varyin the film grown on the substrate surface. Detector crystals cut fromsuch film would be of variable semiconductive properties and their usein a multiple detector configuration presents substantial performanceand circuit design problems. By using the separation gap to flatten thethermal profile lines, films of up to /2 square inches have been grownfrom which a plurality of crystals may be out having substantially thesame semiconductor properties.

Examples of other actual growth conditions which produced high qualityepitaxial films are shown in the following table.

TABLE I Analysis in Growth 1: X-ray Flourescnce Heat Sink Run No. CountsTemperature Hg Cd Te C. 23 MK lll 2 2.5 30.1 47.4 520 24 MK lll 27.627.9 44.5 5l5 25 MK lll 4L8 l9.9 38.3 510 26 MK lll 60.3 7.l 32.6 505Furnace Source Reaction Furnace Temperature C. Coil Separation Hg Cd Te23 MK lll 3l5 370 510 l cm. 24 MK lll 3l5 370 SH) 1 cm. 25 MK lll 3l5370 510 lcm. 26MKlll 315 370 S10 1cm Mixing Furnace Reaction FurnaceTemperature C. (foil 51 (O l 52 23 MK lll 850 800 530 2 4 MK lll 850 800530 2 S MK III 850 800 530 26 MK lll 850 800 530 In all the abovesamples, the substrate was a Cd Te monocrystal of a nominal thickness of10 mils out along the crystallographic plane, the carrier gas washydrogen, and the source materials were 99.9999 percent purity cadmiumand tellurium. The growth times were 2 hours in all cases with a 60cc./min. flow rate through each of the source furnaces 10, 11, and 12.The films produced having substantially uniform composition wereproduced in sizes from V4 to 9% square inches. By making the growthapparatus longer, larger size crystals are obtainable.

While the above examples show use of Cd Te growth substrates, othermaterials, such as Pb Te and Sn Te may be used. Also, while hydrogen gaswas used in all specific examples, argon and argon hydrogen mixtures mayserve as the carrier gas.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the sprite andscope of the invention.

We claim:

1. A method for vapor growing ternary epitaxial films on a substratecomprising the steps of:

forming a gaseous mixture consisting of growth reactant gases fromelements of Il-Vl groups and an inert transport agent;

heating said gaseous mixture to a temperature which prevents binarycombinations of said reactant elements;

forming a sharp thermal gradient proximate the growth surface of saidsubstrate by cooling said substrate to a temperature at whichcondensation of said reactant gases from said mixture occurs, and byheating the regions surrounding said substrate in a manner whichproduces a thermal gradient immediately above the growth surface; and

flowing said mixture through said gradient into contact with saidsubstrate to rapidly supersaturate the growth reactant elements withinsaid mixture to effect film growth of a ternary compound thereon.

2. A method for vapor growing ternary epitaxial films in accordance withclaim 1 in which said gaseous mixture is formed by adding each growthreactant gas to separate streams of said transport agent, followed bycombining said streams into a common stream and flowing said streamthrough a mixing device to produce uniform distribution of saidreactants in said transport agent.

3. A method for vapor growing ternary epitaxial films in accordance withclaim 2 in which said heating to prevent binary combinations of saidreactant gases is done concurrently with said mixing of said separategas streams.

4. A method for vapor growing ternary epitaxial films in accordance withclaim 1 in which said reactant gases consist of elemental mercury,cadmium, and tellurium and said transport agent is selected from a groupconsisting of hydrogen argon, or a mixture thereof.

5. A method for vapor growing ternary epitaxial films in accordance withclaim 1 in which the cooling of said substrate and the heating of theregion surrounding said substrate is done in a manner which produces athermal gradient immediately above the growth surface of said substratehaving equithermal lines substantially parallel with said growthsurface.

$32 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3-619Datum Gerald W. Manley, Philip S. McDermott, Invenmflflmm Ralph J. ElieIt is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

7 On the cover sheet, following the official title [5417 change "7claims, 4 Drawing Figs to read --5 claims, 4 Drawing Figs.-.

Column 1, after the title (lines 1 and 2) and before "Background of theInvention" (line 4) insert the paragraph -The invention herein describedwas made in the course of, or under a contract with the Department ofthe yo I Fligncd and sealed this Qth day of May 1972.

FDWI IRD DLFL'ETCHEH ,JR. ROB RP GOT'I'SCHALK A; testing OfficerCommissioner of Patents

2. A method for vapor growing ternary epitaxial films in accordance withclaim 1 in which said gaseous mixture is formed by adding each growthreactant gas to separate streams of said transport agent, followed bycombining said streams into a common stream and flowing said streamthrough a mixing device to produce uniform distribution of saidreactants in said transport agent.
 3. A method for vapor growing ternaryepitaxial films in accordance with claim 2 in which said heating toprevent binary combinations of said reactant gases is done concurrentlywith said mixing of said separate gas streams.
 4. A method for vaporgrowing ternary epitaxial films in aCcordance with claim 1 in which saidreactant gases consist of elemental mercury, cadmium, and tellurium andsaid transport agent is selected from a group consisting of hydrogenargon, or a mixture thereof.
 5. A method for vapor growing ternaryepitaxial films in accordance with claim 1 in which the cooling of saidsubstrate and the heating of the region surrounding said substrate isdone in a manner which produces a thermal gradient immediately above thegrowth surface of said substrate having equithermal lines substantiallyparallel with said growth surface.